Sensitometric device



Nov. 7, 1950 A. SIMMON SENSITOMETRIC DEVICE Filed March 27, 1948 3 Sheets-Sheet 1 INVENTOR: Alfred Simmer! Ma, f. MM

A TTOR/VE Y.

Nov. 7, 1950 A. SIMMON SENSITOMETRIC DEVICE 3 Sheets-Sheet 2 Filed March 27, 1948 INVENTOR: A/frea 5/711/110/1 Wu 5 WWW A TTORNEY.

Nov. 7, 1950 A. SIMMON SENSITOMETRIC DEVICE 3 Sheets-Sheet 3 Filed March 27, 1948 Fig.6

INVENTOR: Alfred Simmon Wm s. Maw-W Patented Nov. 7, 1950 UNITED STATES PATENT OFF-ICE I 2,528,977 SENSITOMETRIC DEVICE Alfred Simmon, Jackson Heights, N. Y.,assignor to Simmon Brothers, Inc., Long Island City, N. Y., a corporation of New York Application March 27, 1948, Serial No. 17,537 2 Claims. (01. 8814) the device also indicating the circuit in'a more.

or less schematic form;

Fig. 2 is a horizontal cross-sectional view'along the plane of line 2-2' in Fig. 1.;

Fig. S3 is a detailed diagram. of the electrical circuit;

Fig. 4 is a mask forming part of a modifying circuit by which certain photocell currents are renderedproportional to photographic densities; and

Fig. 5 is a schematic view of' a modification ofan illuminating device suitable for opaque test specimens.

Like characters of reference denote similarparts throughout the several views'and the following specification.

Principle An original object and a photographic reproduction made therefrom are scanned simultaneously by synchronized pencils of light in such a way that corresponding points oforiginal and reproduction always receive illumination at the same time. Part of the light reflected or transmitted, depending upon whether one deals with opaque or transparent specimens, is made to impinge upon two photoelectric cells. By means of suitable modifying circuits the photo cell currents are rendered proportional to'photo-' graphic densities, and-voltages corresponding to these currents are impressed upon the hori-'- zontal and vertical deflecting means, respec-- tively, of' a cathode ray tube. The curveappearing upon the-screen of that-cathode ray tube represents then directly the density of each point of the reproduction as a function of' Representation of electrical circuits In the interest'of simplicity; voltage sources have in the following been represented by batteries, but it will be understood that in reality these batteries can be readily replaced in the conventional manner by transformers, rectifiers, and condensers. Linear sweep circuits or saw tooth wave voltage generators of this invention. Supply circuits for the various filaments for the cathode ray tubes have, been omitted. These filaments are, in reality, either fed from small batteries or from filament All circuits used in this device transformers. are extremely sensitive to small voltage fluctuations and, therefore, automatic voltage regulators or stabilizers must be used in order to' render all voltages independent of small changes; In the interestof clarity shown in thei of the line voltage. these stabilizers have not been diagram of Fig. 3.

b tructure In its preferred embodiment the'device comprises a cabinet-like housing 20., Fig. 3, on top of which is mounted a smaller housing 2! containing the two illuminating devices, two containers 2'2 and 22" forthe two photo-electric,

cells, and two light collecting elements 23 and 23" which connect the illuminating devices with A slot 2G is provided the photocell housings. which permits the insertion of both the original and the photographic reproduction. It' is pref-l erable to add mechanical adjusting means by which the reproduction and the original can be shifted relative to each other until the two scanning beams of the illuminating device will strike corresponding points at all times. adjusting means should preferably comprise means to shift either the original or the reproduction in two directions which are at right an-' gles to each other and should also permit a certain amount of rotation. Mechanical adjusting means of this nature are well known .and can be found, for example, on microscope stages and machine tools. Since the exact design of these adjusting means has no bearing upon the The cabinet 20' serves notonly as a base for the illuminating means and the photoelectric cells, but also as a housing within which the component parts of the various circuits can be mounted. The most important component part is a cathode ray. tube 25 which is so mounted that its screen i visible through a correspond-- ing aperture in the front wall of the cabinet;-

are also shown only in block form since their detailed construction is well knownand forms no part These 20. It is on this screen that later the curve appears which represents the function of the reproduction versus the original density.

The other component parts contained in the cabinet are shown schematically since their exact position in this cabinet is unimportant. These component parts can be conveniently classified as the supply circuits 26 for the illuminating means, the supply circuits 2'! for the photoelectric cells, and modifying circuits 28. connecting the two. scribed in detail in a later paragraph; 1

Original object and photographic reproduction Both the original object and the reproduction made therefrom may be either opaque or transparent. For the investigation of negative materials, a transparent original is to be preferred since only in this manner can a sufficiently wide range of densities be provided, and the photographic reproduction made upon negative material is, of course, itself transparent. 1 The reproduction may be made from the original in this case by a contact print or the original may be photographed with a, camera upon the negative material to be investigated. In the latter case, the original, of course, has to be placed in front of a suitable source of light, i. e., illuminated from behind. An opaque original can be used if the contrast range to be investigated need not be.

very large and, if the material to be investigated is photographic positive material such as bromide paper, the reproduction will also be opaque. Depending upon whether transparent or opaque reproductions or originals areused, different optical systems for the illuminating means must Illuminating and scanning means The object and its reproduction are respe'cf tively illuminated by synchronously moving 'peni oils of light. The light may be provided by any.

convenient source such as incandescent or carbon arc lamps and, likewise, scanning means of any convenient design are applicable such as, for

example, rotating discs with suitable apertures on; their periphery, or rotating or otherwise moving mirrors or lenses. In practice I prefer, however, to use cathode ray tubes because in this manner all mechanically moving parts are avoided and because the light output of a cathode ray tube can be most conveniently regulated by changing the 'voltage impressed upon a control. grid. Therefore, illuminating and scanning means comprising two cathode ray tubes have been shown in the drawings, and will be described in the following paragraph.

Since the illuminating and scanning means used for the original and reproduction, respectively, are substantially identical, corresponding parts will be denoted by the same figures to which, in the case of the original, and in thecase of the reproduction, has been afilxed. The same practice has been followed in the two following paragraphs describing the two photocells and their supply circuits and the two modifying circuits connected therewith, respectively.

Referring to Figs. 1 and 2, the two cathode ray tubes used as illuminating means are 30 and 30". Their component elements are shown in These circuits will be 0le-'v 4 more detail in the diagram in Fig. 3 and comprise filaments 3I' and 3|", indirectly heated cathodes 32', 32", control grids 33', 33", two focusing elements 34, 34", 35', 35 and two pairs of deflection plates 36', 36 and 31', 31". In front of the tubes are mounted in the usual manner light emitting fluorescent screens 38' and 38". The' orig'ial'. 40" and the: reproduction 40" are mounted in front of their respective tubes, it being assumed for the time being that both original and reproduction are transparencies. The modification necessary to accommodate opaque specimens willbe described later.

The supply circuitfor these two tubes is con- "ventional, comprising batteries 4| and 4|", contubes in the usual manner as shown in Fig. 3.

For the purposes of this invention, only one set of deflection plates is really needed, and the second set has merely been shown because it is contemplated to use commercially available tubes which are always equipped withtwo sets. Therefore, the two pairs 36' and 3B" are connected to the ground or are in any other way rendered ineffective. The remaining plates 31' and 3l" are connected to a common sweep circuit 43. Due to this connection, the luminous spots upon the screens 38' and 38." are driven synchronously by this common sweep, circuit 43. The deflection plates 31' and 31" are connected to additional biasing means which comprise batteries 46, 46" and potentiometers .41, 41". The purpose of this arrangement is to make it possible for the operator to adjust the starting point of the sweep circuit.

The control grids 33' and 33" assume a certain negative potential relative to their respective cathode 32 and 32".

Photoelectric cells and supply circuits The two photoelectric cells 50' and 50" may be of any desired type known in the art, but in practice I prefer the so-called photoelectric multiplier since additional amplifiers are usually unnecessary withthis type of tube. Referring to Fig. 3, these cells comprise glass vessels 5| and 5|" within which the photo sensitive cathodes 52' and 52" and nine additional'electrodes 53' and 53" are mounted. f These elements are connected to respective points :of the potentiometers 54', 54 upon which a voltage is impressed by batteries 55, 55 or equivalent sources of voltage. The cathodes 52', 52" are connected to the most negative pointsat'the extreme right of the potentiometers, and the last electrodes are connected to the extreme left point of their respective potentiometer so that they receive the most positive potential. The wire connecting the left end of each potentiometer to the last electrode is interrupted and two resistors B0", 6!, 6|" are inserted in this circuit. During operation, voltages, depending upon the resistance values of these resistors and upon the current circulating in the last loop ofthe photoelectric multiplier circuit, are built up across these resistors.

The distance of the two photocells from their respective transparencies and from the respec- This negative bias con- 7 sists of two parts, a fixed part which corresponds 5i tivexscreens: of the cathode":ray 'tubessmustizbe; sufficiently large;- so that allipoints-"ofi'said trans-2 parency havesubstantially" the: same 1 distance from said-cell.

' Thelightreaching the photoelectricicells from theiscreen of the.cathode ray tubes.:can: bem'ag-i nifiedby placing suitably shaped light. collecting. elements between. screen-and photocell;v These: light collecting elements may, for example; be. truncated cones or pyramids with arrrinner refleeting surface, or they may-.be-solid'bodies: made from glass or transparent plastic in which case: the light would be directed toward 'the photocell by total reflection along the-boundaries? M odi jy-i'ng circuit-general. principle The currents passing photocells" 5l",. 5|," are proportional to the light intensities passingpthe specimens40' and 40,; i. e'., thereproductionand. the. original,j.respectively. Since it is desiredtto'. compare densities rather than lightjintensities; meansmust be provided to produce .currents or: corresponding voltages which are. proportional. to the densities to be measured. This is the .purpose of the modifying. circuits... Therefore, thei modifying, circuits may. comprisgsuitttbl'e' net'- works which. have .a non-linear response .in such. a, .way, that the photocellcurrent's fed. into them areproportionalf to the intensity. of the. imping: ing.- light and'thatl the currents. or voltages delivered .by, these modifying circuitsware proportionaltothe corresponding densities. 'Manynet. works with non-linearresponse. are known, com.- prisingnon-linear resistors; vacuum. tubes: or similar elements, but. regardless. .ofIthe detailed designs, such. an arrangementlwould suffer-.fromv the. very serious disadvantage that. the photocell. currents themselves would have to lie/proportional to the light intensities and, if .oneimeasures densities up-to 3. theli'ght" intensities and there with the photocell currents willlvarj in the pro.- portion of. 1 1000; Ittis very. inconvenient if .-not' impossible. to design circuits. which. willwsatise factoril'y cover such an exceedingly. wide ra'ngel. I, prefer, therefore, .to. solve th'e'problem-in the following manner: The intensity ofothe.light. im.-- pinging. any. given point of the two specimens!!!" and 40-" is modulated.inaccordance. to; the trans:- initted light intensity. This is. done by the photocell. current itself. and a. voltagewhich-is .a function of. said photocell. current is impressed. uponthe grid of the cathodeiray tube. changing: its light output. In this manner. the fluctuations of... the light impinging upon.- the photoelectric cells 5| and. 5| arereduced; and-itis eVen .p0ssible to. modulate the lightintensitiesinusuchia manner that theresulting photocell currents. are nowdirectly proportional to. density values. The modifying-circuit. which accomplishes this func= tion is described in.the next paragraph.v

Modifying -circuits mathematial basis" I The photocell output currents or,.morei specifie; cally, the Voltages impressed... upon resistances 60.; .60 are fed 'into' two" modifying circuits: The output voltages: of "these: circuitsiarerthen' impressed upon the gridsof the respectivei'cath ode; ray, tubes and: used; there; to modulate; :the:

brightness of the: luminous. spot 'formedi'on thez screen. It is, therefore; clear'that'ithe'modifying circuits must" deliver outputtvoltages which. are definite mathematical. functions .of their inputcurrents on voltages; This;mathematicaliiunce; tion .willbelderived ing'the. followingparagraphz'.

r veniently expres'sedin micro amps (pA).

I is the current passing the cathoderraytube"atL any'given instance, and Imxis the maximum'peremissible cathode ray tube current. This value:-

may be determined either. as the maximumcurrent that: the screen of the tube will withstandi without deterioration, or it may be the maximum value for which the linear relationship between cathode. ray tube current and grid bias seunoms: true, or it may be determined b any other-cone venient consideration. Both I and Imaxare' 'cone is the additional negative voltage impressed upon the control grids '33" and 33" which is added to the voltage impressed upon these grids by the left end of potentiometers 42 and 42". In other'words, the left ends of potentiometers 42', 42" impose a: certain constant minimum negative voltage uponthe grids at all times and the voltage e"which=-is" the output voltage of the modifying circuits will be added thereto. 2 is expressed in volts. a is a constant which depends upon the characteristics of the individual cathode ray tube and which has a dimension volts The light intensity-on screens =38', 38" i. e., before .the light. has passed the reproduction orthe original. object, is again withinreasonablei'limitsdirectly proportional to the cathode--ray -tube" current or L1=bI (2) L1 is expressed in foot candles, or any corresponding metric dimension, and b is a constant again depending-upon the characteristic of the cathode ray tube, measured in The lightintensity after the light has passed the transparency, is expressed by where'Dis the density of' the reproduction ororiginal at the point which, at any -'given dis= tance, is illuminated b the luminous spotof'thecathode ray tube.

The light intensity in the plane of-"the photo sensitive'cathodeof the photoelectric cells 5 i I, 51 is proportional toLzbut, ofcourse, considerablysmaller, depending upon the distance between the two tubes:

- L3=dL2 (4) dis a dimensionless constant.- I

The. photocell current is again: within wide: limits propo'rt-ional to the light impinging upon the photoelectric cell or w i: in (5):

where f is a constant denoting the light sensitivity of the photoelectric. cell in.

foot candles V I shallnow impose the condition that' the modifying circuit.shallidelivene as afunction oi i, i. e., outputzvoltage as ta' function. of input current; in

such away-that differences of photocell currents h isa proportionalityiactor which has the di-Fi mension .of' acurrent and which is: expressed in ,uAIs; i u

D1- and Dzare the photographic densities of two points of thfspecimemand i1 and i2 arefthe correspondingphotocell'currents; i. e.,the currents whichgare'measured when the luminous spot of the cathode ray tube is placed, respectively, behind the-two points with the two densities D1 and D2.

Equations 1, 2,3, 4 and 5 can be combined and solved for ,D with the following result:

I (I -ad] V ,(7)

Eor,D= Di-there is, of course, i=z'1 and e=e and likewise forfD -Dz there is i=z'z and 6:62; By

substituting. these values, I arrive at thefollowing expressionfor D2D1:

'Loc i (8 L1 DzD1 can also be computed from Equation 6:

In "the two Equations 8 and 9, 82 becomes zero, andi'zbecornes'i m fOT DZ AmaX, where Amax iS the highest density within the measuring range of the device. Amax usually'need not exceed the value 3.

min

These two-equations can be combined and solved for 121 with the following result:

' min This is the mathematical function according to which the modifying circuits have to work,- i. e., When-the spot ofa cathode ray tube passes a point with the density D1, a photocell current i1 will be generated which, by the modifying circuit, will'be changed into a voltage e1 which is then impressed,

"in addition to a constant negative bias, upon the i ,=bdfI,,,,,,-10 Modifying circuit-prejerred design The design of the modifying circuit itself is basically of no importance and any circuit which will modify an input current into an output volt-v age. according to Formula 10 will be satisfactory. Circuits of this type are frequently networks using at least one non-linear resistance or'impe'd'- imin can be com-' ancewelement such asoneof the so called-semis: conductors or a deviceusing a saturated ironcore; or one'of the various ,types-ofvacuum tubes. "A; preferred modifying circuit, however, which is in many respectsuperior to these networks, I has been disclosed in my co-pending application Serial No. 791,439'now Patent No. 2,474,380 and willbe described .asa preferred example in the follow- A modifying circuit built accordingly consistsof threeprincipal'parts, cathode ray tubes 10, 10C, stationary masks H, 'H and photoelectric cells 12', 12".; It is emphasized that the cathode ray tubes 10,\]0" and the;photoelectric cells '12, 12!.

.are in no way identical with elements 30, .30" and 51', 5| which have been described above. They are entirely independent therefrom and perform entirely different functions- The: broad principle of the modifying circuit is thatin some suitable manner a luminous line is formed on the screen of the second cathode ray tubes 10', 10 that the incoming signal, which in this case is the current passing the first photocells 5| 5| is used to deflect this line .ina direction atright angles to itself and that thereby, part of the light emitted by this line is cut 011' by the stationary masks placed in front of the luminous screen of tubes 1 0, 10". The light p'ermitted'to pass these masks is then made to impinge upon the second photoelectric cellsfIZ',

12" forming part of the modifying circuit, but,

not upon tubes 5|, 5l' The current passing these second photoelectric cells 12, 12 or, more precisely, the corresponding voltage impressed upon a resistor of suita ble magnitude, is then suppliedas additional bias to the control grids 33', 33; of ,.the' original cathode ray tubes 30, 307 hich', scanthespecimgn in the manner c d; v These circuits are shown in detail as the right half of Fig. "The cathode ray tubes 103 10, 7 cQntainLfiIan entsJQ, 14 cathodes 15', .15f

heatedtherebyfcontrol grids 1a, 16", twofocu s'g ing members 'I.'! 'IIj and'IB, 1B" includingsec-' ond control grids two'pairs of deflection-plates 19],, lgf' anddiil, ,80" and a screen capablev of lightiemittancel H I A luminous linecan'be formedupon this sc reen by any desired means, for example, by giving the. focusing elements 11, l1" and 18', 18 a proper configuration. Ipr'efer, however, to use a con-J ventional cathode ray tube which forms a luminous spotupon the screen which is trans formed into a line by means of sweeps circuits 8 l',"8 l operatively connected to the deflection plates 80,, This sweep circuit is a saw' tooth wave generator of the customary type which has a' high frequency compared to the frequeneylof thefs'ignal impressed upon'the other pair ofdei lection'platesl It can be seen that the other set'of deflection plates 19', 19" is operative y connectedto; resistance elements 60!,- 60" upon which a, voltage proportional to the current passing the-:first photocells 5|, 5|" is impressed. Both 'se'ts ofdefletion'plates '19','19" and B0; 80'""are biasedby'batteries 82', 82" and 83', 83*" which are Iconnected' respectively; to 'potenti oni'eters 84-; 84" and 85','85". By'means ofthis arrangement; the starting points ofthe two plates (9",

il'elementswithin the ic'athode ray tubes inl such a 'wayrthat the :controlrgrids 1 6 I assume a ifixed snegaitive :potential with respect to the =cathodes l'," l 5, but that- 1 1, 11 have a positive potential with r'espectto 'lfl, 15", 1 8', '18 ialpositive potential with respect to :11, 11", ahd the screen finally a positiv e -potential with retspecttoJB', 18".

' In frontof'the screen, and preferably immediately adjacent ther'eto-are masks'll, TI" "which are again shown inF-ig. =4. Each mask has an ape'rture which directlyrepresents the mathematical function-expressed in Formula 1'0. The mask is a ith'in'plate made o'f opaque material such as black paper and sheet metalfland its vertica'l width varies-as a function of the hori- .zontal distancelfrom a point of reference 9'0 in accordance with'Formula 10. 'For convenience, the lower borderline of this aperture is made-a straight line, but this is not necessary and both the upper and lower bordersmay becurved as long as "the vertical width of the-aperture is the desired function of the horizontal distance from the point of reference 90. It can'be-Seen that only that part of-the .-ligh temitted by the luminous line 9I which is behind said aperture can pass and that the other parts which are shown in 'dotted lines above and below this portion-are blocked off. The light impinging upon the "second photoelectric cells 12, I2 is, therefore, proportional to the length of that portion of line 91 which appears'behind the aperture or is a direct function of the shape'of said aperture, in other words, varies in accordance With'Formula '10, provided the aperture is 'fabricated correctly.

The respective distances 'between photoelectric cells 1 22 12" and cathode "ray 'tubes 101,10" must, of course, be large enough so that all points of the luminous line have substantially the-same distance from the photo sensitive cathode of the photoelectric cells I2, 12''. These cells may again be of any desired design and I have again shown photoelectric multiplier tubes because then complicated amplifiers can be avoided. Each cell again has photo sensitive cathodes I00, I00, and nine additional electrodes IOI, IN". The supply circuit comprises batteries I02", I02, and potentiometers I03,

I03, the various points of which are in the usual manner connected,re spe'ctively to elements I00, I00 and I0I, IOI. The wire between the'extreme left point of each potentiometer I03, I03 and the last electrode is again interrupted to receive resistance elements I04, I04. The voltage impressed upon these resistance elements by the current passing it represents directly the value e1 'used in Formula 10. e1 is then by means of two wires I05", I05 and I00, I06" fed back into the grid control of the first cathode ray tubes 30', 30", 'i. e., voltage 61 is added to the constant negative bias which control grids 33, 33 have with respect to cathodes '32, 32".

If the luminous line upon the screen'of cathode ray tubes 10, 10 is formed in the preferred manner'by' a voltage of high frequency impressed by saw tooth wave generators 81', 8I upon deflection plates 80, 80," means'must be provided to keep the output current'or voltage of the photoelectric cells 12', I2 from being modulated accordingly. This can be done simply by connecting condensers I01, I 01", shown in do'tted 1ines, of suitable magnitude across points I08,

I08" and I09, I09, or'a more complete filter formed in the usual manner of a network of capacitances and 'inductanc'es can be connected topoin ts 108 08"" and I09, :A fil t'r this ty pe' liasnot been shown in detail si-nce ts design 'is "-well known and f'o'rms no part or "this invention. If, however, the lumino'usfil-ine uponscreen of tubes-104,10" is formed byother means, ror example, by the proper configuration of'the focusing elements 'II,"I'I andl8,I.8"--, this filter means can be dispensed with.

"Recordin cathode ray tube If the lightoutput-of the c'athode-raytube which illuminates and scans the original and reproduction, respectively, are modulated by the output voltage ozf-i the modifying circuit in the manner:describedain'theprevious'paragraphydif 'fel encesc'f iphotocell currents are rendered di rectlypr'ciportionalto di'fierenc'es 'of densities "or, accordin to Equatio'n fi (3f ultimate interest however, are not density'idifferences' but absolute values 'of densities, and these'ar'e obtained from 'a network which com"- prises resistors "0| -6-I "carryingthe "photocell current of cells 5I.:,'"5I and -which is in series with po'tentio'meters -"I*II'., :I?II" 'each of :whi'ch, riniturm receives :a volt'age"Trom batteries or"o'the'r sourceof voltage I I0, 1 I 0. The connection'zof zthese'ibatteries'must be so' chosen'th'at'the polarity ofpotentiometer :I'I I :is opposed to that of the resistors 61 ';--0 I or, in other words, the voltage between "the sliding 'cont'a'cts I'I2, I I2 of 'Tth'e potentiometers:andipdints I I3, I13 is the ifference of the respective voltages impressed upon 'I'IlI.,'"I'I=I" and '6I",""6'I". The function otth'is network can be explained as follows:

Assuming that'the resistance of"BI", '6' I is R, andtha't' the voltages corresponding to photo-- "cell "currents 11,12 :are :and'mz; Equation '6- can be transformed to read:

This -means that i f I impress -a voltageequal-to Emax between points-H2, H 2 and -II-4.,-.-'l M which isoppUsed to the voltage impressed upon SI, SI, that-then the voltage between .points I I3, -I I3" and 2', I I2 is directly proportional to the density D2 :of thatcp'oi'nt "which vajtmny given distance is .illuminated by -the scanning beam of the cathode ray tubes "30,30.

'If one has transparent test 'specimensi such as presupposed up'to this time, the adjustment ef the 'slidingfcontacts H2 II I2 of potentiometers I I I 'I I is particularly -'easy be'cause all one has to: do is to remove the test specimens '00, 10 altogether-so that one :has zero density, and then adjust i'sli'ding contacts I=I2'.,' I I 2 until the volt:- age betweentpoints lI-3'.,II'3 and 112, "112 becomeszerm With :ian ropaque' testspecimen: this 'ad'j ustn'lent is -:a littl'eimorei difficult because even a completely unexposed white piece of paper is not or-zero density ibecause'it 'refiect's less than I0O% of the incident light. LHowever, this reflectivity can be measured by other well known :meaiis, and-the devicefcan then be uste diaccordinglys making. proper" llow'an'ce'for the faet' that the maximum reflection nla "-be, "for exam le, of the generalorder 'o f 90%. 1

Contacts *IfIZ", H'2"'-are connected-by wires H5, I I 5"t'o the defiectioniplates I20, I20" nd I 2 I -I 21"" *of the recording "cathode ray tube-12 2;

:fThis cathode ray tube. is of conventionaldesign containing, in the usual manner, a glass vessel, 9. fllament, a cathode, focusing members, control grid, and aluminous screen. These elements are connected in the usual manner to taps of a potentiometer I23 fed by a battery I24. Biasing means are provided for the deflection plates comprising Operation .The function of .the device will be obvious from the foregoing description. The test specimens of .40, 40" are scanned and illuminated .by the moving luminous spot appearing upon the screens of cathode ray tubes 30', 30". Light transmitted by these test specimens impinges, respectively, upon photoelectric cells 5I". The current passing these photocells as a result of the incident light, is modified by the modifying circuitwhich. in a preferred embodiment of this invention, comprises second cathode ray tubes 102.10", and second photoelectric cells I2, 12". Between the two last named elements, there are masks II, II", and due to the coaction of the three last named parts, i. e., cathode ray tubes I0, photoelectric tubes I2, 12" and masks II, and 'II, output resistors I04, I04" which, when fed back into the control grids 33, 33" of the two original cathode ray tubes 30, 30", modulates their light output in such a way that differences of the currents passing the photocells 5|, 5|" are rendered directly proportional to density differences of the test specimens 40, 40". The networks comprising resistors 6|, 6|" and potentiometers I I I, I I I" convert these differences into voltages which are proportional tojabsolute density values. These voltages proportional to absolute density values are then impressed simultaneously upon the horizontal and vertical deflection plates I, I20", respectively, of the recording cathode ray tube I22. As a result of this arrangement there appears upon the screen of this last named cathode ray tube a luminous curve which is schematically shown in Fig. 1 and'which directly represents the density D1 of points of the reproduction asa'function of the densities D0 of the corresponding points of the original.

Opaque test specimens It was pre-supposed up to this point. that the test specimens, i. e., the original as well as the reproduction were transparencies. This will be true whenever the properties of films and plates suitable for photographic negatives are investigated. However, the investigation of opaque materials, such as bromide paper,'is equally important, and in this case, I have to modify the arrangement slightly in order to. adapt it to opaque specimens which will have to be tested with reflected rather than with transmitted light. Such an arrangement is schematically shown in Fig. 5. A lens I is placed at a suitable distance in front of the luminescent screen of the cathode ray tube 30 and projects an image of the luminescent spot appearing on the screen of said tube upon the opaque specimen 40' which may be either the original or the reproduction obtained therefrom. Opposite the test specimen II at a suitable distance is a photoelectric cell 5I which isadapted to accept part of the light diffusely reflected by 40'. Since unavoidable light losses a voltage is impressed upon the .areincurred by the projection and reflecti'ortiit :may be desirable to use more than one photoelectric cell and a second one has been indicated in Fig. 5 in dotted lines. It is important that the photoelectric cells be positioned in such a way that they intercept only diffusely reflected light and not a specularly reflected beam which, in the case of glossy photographic paper, may be of appreciable intensity. This, of course, would falsify'the results completely.

It is advisable to place a shield I3I between .the cathode ray tube 30' and the lens I30 which prevents light from the luminous screen of the cathode ray tube to reach the photoelectric cells directly.

Instead of the simple lens I30 which has been shown more or less schematically, it is, of course, possible to use more complex optical systems. A particularly efficient system has been developed lately which follows the so-called Schmitt camera and which consists of a light collecting reflector and an aspherical correction plate. Since a system of this type forms no part of the invention, it has not been shown.

Modifications It has already been pointed out that many component parts of this invention can be changed freely. This has particularly been mentioned with regard to the modifying circuit, but other changes of this nature will occur readily to anybody skilled in the art without departing from the principles or sacrificing any of the 7 advantages of the invention as defined in the appended claims. For example, while all cathode ray tubes have consistentl been shown equipped with electrostatic deflection plates, it is also possible to use electromagnetic deflection means.

What I claim as new is:

1. A device for plotting the density of points of a photographic reproduction as a function of the density of the corresponding points of an original object, comprising means to illuminate said reproduction and said original, respectively, by two moving pencils of light, including means to synchronize the movements of said pencilsof light whereby corresponding points of reproduction and original are always illuminated simultaneously, two photoelectric cells including electric circuits operatively connected therewith, and including means causing said cells to receive light coming from said two pencils and modified by the respective densities of said reproduction and said original, means to render the output voltages of said photocell circuits proportional to the respective densities of said reproduction and said original, a cathode ray tubeincluding means to form a beam of electrons, first means to deflect said beam of electrons in one direction, second means to deflect said beam in a direction perpendicular to said first named direction, means to accelerate said electrons, and a screen capable of light emittance when struck by electrons, and means to actuate said two deflecting means, re: spectively, by the outputvoltages of said two photocell circuits, said means to illuminate said reproduction and said original, respectively, comprising two cathoderay tubes, eachincluding means to form a beam. of electrons, means, -in-.- eluding a grid, to controlthe number of said'electrons, means to deflect said beamof electrons,

means to accelerate said relect rons, and a screen capable of light emittance when struck by electrons, and said means torender the output volt-- ages of said-photocell currentsproportional to;

the respective densities of the reproduction and of the original, comprising two modifying circuits, each fed by the respective current of one of said photoceils, and delivering an output voltage substantially according to the formula i i.10 Ts' where e is the output voltage of one of said circuits,

1' is the corresponding photocell current, and

A, B and C are constants,

and means to impress the two output voltages, respectively, upon the two grids of said cathode ray tubes, whereby the brightness of the two luminous points on the two respective screens is automatically modulated in accordance with the respective densities of the two corresponding points of reproduction and original which are illuminated at any given instance.

2. A device for plotting the density of points of a photographic reproduction as a function of the density of the corresponding points of an original object, comprising means to illuminate said reproduction and said original, respectively, by two moving pencils of light, including means to synchronize the movements of said pencils of light whereby corresponding points of reproduction and original are always illuminated simultaneously, two photoelectric cells including electric circuits operatively connected therewith, and including means causing said cells to receive light coming from said two pencils and modified by the respective densities of said reproduction and said original, means to render the output voltages of said photocell circuits proportional to the respective densities of said reproduction and said original, a cathode ray tube including means to form a beam of electrons, first means to deflect said beam of electrons in one direction, second means to deflect said beam in a direction perpendicular to said first named direction, means to accelerate said electrons, and a screen capable of light emittance when struck by electrons, and means to actuate said two deflecting means, respectively, by the output voltages of said two photocell circuits, said means to illuminate said reproduction and said original, respectively, comprising two cathode ray tubes, each including means to 4 fore, and with a screen capable of light emittance,

a supply circuit for said independent cathode ray tube, including means to excite a straight line on said screen to light emittance, means to deflect said line substantially at right angles to itself inaccordance with the current passing the photocell connected to this modifying circuit, an apertured mask made from opaque material placed in front of the screen of said cathode ray tube, the width of said aperture in a direction substantially parallel to said light emitting line varying substantially according to the formula Y isthe width of said aperture,

X the distance from a point of reference, in a direction at right angles to said light emitting line, and D, E and F are constants,

a photoelectric cell, independent of the cells named before, placed in front of said mask and connected to a supply circuit, and a resistor adapted to be passed by the current passing said independent photocell, the voltage thereby impressed upon said resistor being used to bias the control grid of one of the first named cathode ray tubes which are used as illuminating means.

ALFRED SIMMON.

where REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,324,270 Schesman July 13, 1943 2,349,687 Williams May 23, 1944 2,375,966 Valensi May 15, 1945 2,413,706 Gunderson Jan. 7, 1947 2,434,561 Hardy et a1. "Jan. 13, 1948 

